Extruded dielectric high-voltage power cables may fail prematurely in service for a number of different reasons. These reasons include mechanical damage to the cable in shipment, storage or handling in the field and during installation. Another reason is conductor and/or insulation semiconducting shields that exhibit excessive electrical resistivity, particularly after load cycling of the cable in service and excessive corrosion of uncovered outer concentric neutral wires.
Other failues result from significant imperfections in the insulation structure of the cable in the form of skips and projections at the conductor shield, voids and contaminants in the insulation and irregularities and poor adhesion at the insulation shield interface with the insulation which may not be detectable at the factory by present industry tests. Still another cause of failure is the development and propagation of electrochemical trees, resulting from moisture ingress into the insulation and the presence of imperfections and high electrical stress in the region of the imperfections.
Installed high-voltage, extruded dielectric, power cable systems have been tested in the past by imposing a high-voltage direct current on the system. Such tests detected some of the imperfections; but were ineffective in detecting other causes of premature cable system failures,
AC high-voltage acceptance and maintenance tests have not been practiced to any significant extent because of the heavy weight and bulk of the equipment required due principally to the relatively high capacitive current drawn by the cable at the voltage required to yield meaningful results. Conventional dissipation factor bridges used for factory tests require that the cable insulation shield be electrically insulated from ground, whereas installed cables normally have the shields permanently grounded. Partial discharge tests (corona) on long lengths of cable, with conventional equipment, require a corona-free 60 Hz high-voltage power supply with large kVA capacity. Such units are too heavy to conveniently transport to field test locations. Furthermore, if the normal 60 Hz power is used in the field to energize the standard partial charge detectors, there is the extremely difficult task of filtering out the intense parasitic interferences present in these power lines.
With this invention, the partial discharge and dissipation factor bridges are modified to operate with cable systems having the outer shield of the cable grounded, as is the case with cable systems installed in the field.
This invention preferably includes the additional characteristic of including a power supply which operates at a frequency of 0.1 Hz. At this frequency, the kVA capacity needed to charge long lengths of cable for partial discharge tests is only 1/600th of the conventional 60 Hz supply. This makes the size and weight of such power supply small enough to be portable for transportation to field installations.
Other features and advantages of the invention will appear or be pointed out as the description proceeds.